Date: Wed, 16 Jan 2002 22:50:16 +0100 From: Bernd Walter <ticso@cicely8.cicely.de> To: Terry Lambert <tlambert2@mindspring.com> Cc: Trevor Johnson <trevor@jpj.net>, Riccardo.Veraldi@fi.infn.it, freebsd-alpha@FreeBSD.ORG Subject: Re: DEC3000/300 Message-ID: <20020116225016.N50371@cicely8.cicely.de> In-Reply-To: <3C45EAE6.CF12B5C5@mindspring.com> References: <20020116023458.E17814-100000@blues.jpj.net> <3C45DF84.5990A4E4@mindspring.com> <20020116214931.M50371@cicely8.cicely.de> <3C45EAE6.CF12B5C5@mindspring.com>
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On Wed, Jan 16, 2002 at 01:04:38PM -0800, Terry Lambert wrote:
> Bernd Walter wrote:
> > > A real NULL-modem cable looks like:
> > >
> > > TX -------- RX
> > > RX -------- TX
> > > CTS -------- RTS
> > > RTS -------- CTS
> > > GND -------- GND (signal ground, not chassis ground)
> > > DCD -------- DTR
> > > DTR -------- DCD
> > > GND -------- (chassis ground on one end to all other wires)
> >
> > I usually additionaly bridge DSR with DCD on both sides just in
> > case I need to use it for designs requiring DSR.
>
> I considered that, but these are DTE interfaces, not DCE, so it
> makes no sense.
>
> If I knew it was DTE<->DCE or DCE<->DTE, I would have tied DSR
> on one side to DSR and DTR on the other for both sides, per
>
> Technical Aspects of Data Communications
> John McNamara
> Digital Press
> ASIN: 1555581110
Think of DTE<->DTE with CDSR_OFLOW.
It's unusual for Unix systems but some others do.
> (I don't know why I always think this was by McKneely; in any
> case, for $5 used on Amazon, this is a cheap way to get some
> neat information, including the Bell 103 and 202 standards,
> which are expensive to get elsewhere).
>
> As it is, you could end up with +11v <-> +11v, and with the
> signal grounds tied and the chassis ground untied, you could
> get a ground loop (best case) or cook a TTL chip (worst case,
> assuming a multiport chip or multiple connections per pinout
> internally).
I only see single output connected to one or two inputs.
What I meant is (symetric of course):
DTR --------o--DSR
|
`--DCD
I havn't thought about using the cable accidantly in a different
environment, but fortunately modern interfaces are protected in
many ways so the worst case you should get is a non working
connection.
A simple diode to protect from current backflow and a current
limitation on at least one side is all you need to handle this case.
--
B.Walter COSMO-Project http://www.cosmo-project.de
ticso@cicely.de Usergroup info@cosmo-project.de
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